<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 250px;
            height: 35px;
            border: 1px solid yellowgreen;
            position: relative;

        }
        div::after{
            content: "";
            position: absolute;
            top:8px;
            right: 10px;
            width: 10px;
            height: 10px;
            border-right:1px solid #000;
            border-bottom:1px solid #000;
            transform:rotate(45deg);
            transition:all 0.1s;
        }
        /* 鼠标经过div 里边三角旋转 */
        div:hover::after{
            transform:rotate(225deg);
        }
    </style>
</head>
<body>
    <div></div>
</body>
</html>